No. 538] REPRODUCTION IN THE BROWN RAT 629 



seen that only one pregnancy occurred as a result of 

 each six times of mating. The table indicates, also, 

 either that the mating periods come irregularly, or that 

 the females come in heat about every five days. 



I am convinced that odor is the final test for sex rec- 

 ognition. When rats are placed together, the males as- 

 sume an aggressive attitude and fight all those that op- 

 pose them. The females may, usually do, for a little 

 while, resist the males ; but they soon yield. They then 

 lie on their backs entirely passive, while the males nose 

 them about the head and smell of their genital organs. 

 This attitude is frequently taken by pregnant females. 

 The males in such cases have shown their superiority 

 and the females recognize it. From now on there is little 

 fighting on the part of the males, and afterwards they 

 will often permit themselves to be severely punished by 

 the females without injuring them. Copulation does 

 not usually follow the above proceedings. 



When males meet, there is a battle royal until one is 

 recognized victor. The conquered, then, tries to elude 

 the stronger, and will not submit to be smelt of as a 

 female. Often males smell of each other, probably de- 

 termining one another's sex, before beginning the fight. 

 I am sure there is no sex recognition when the fight is 

 begun at once, for the females are treated in the same 

 way. 



There is very little courtship among the rats. The 

 male is absolutely silent and the female almost so during 

 the period of heat. A pair which had been together for 

 several months, were seen to arouse from their sleep at 

 five-thirty o'clock, p.m., and begin copulating at once 

 without any preliminaries at all. They had not mated 

 previously during the day. 



When a female resists the advances of the male, she 



kicking him auav with !u r hind foot; or she may lie on 

 her back, as previously described. On the other hand, a 

 female in full heat is the more active of the two. In one 



